1. Field of the Invention
The invention relates generally to the design of safety valves for use in closing off portions of a well during an emergency.
2. Description of the Related Art
Safety valves are used to close off portions of a live well in the event of an emergency, such as a blow out. Well-based safety valves are typically of two conventional styles: flapper valve and ball-style valve. Flapper valves have a substantially-disc-shaped flapper member that is pivotably secured to the surrounding string of tubing. The flapper valve is spring-biased toward a closed position, but during normal operation, is held in an open position by an axially moveable flow tube. When it is desired to close the flapper valve, the flow tube is moved axially within the tubing string so that it no longer holds the flapper valve in an open position. The spring then urges the flapper member to the closed position wherein it blocks fluid flow through the flowbore. The flapper valve can be reopened by axially moving the flow tube to urge the flapper member back into the open position. The ability to rapidly close a flapper valve has made it a popular choice as a safety valve in production string arrangements.
Ball valves have a generally spherical closure member that can be rotated within a housing to permit or completely block fluid flow across the housing. The ability to completely close off flow through a flowbore has made ball valves popular as a safety valve in pipelines, and to some extent in production tubing strings.
Unfortunately flapper and ball-type valve assemblies are substantially formed of mechanical, largely metallic, components that are inherently prone to risk of failure over time due to corrosion or mechanical breakdown. The environs of a wellbore or pipeline are extremely hostile to such devices due to the presence of extreme temperatures and pressures and corrosive and caustic chemicals.
The present invention addresses the problems of the prior art.